The present invention relates to an optoelectronic scanning apparatus which may be advantageously incorporated into a facsimile reader or the like.
Photosensor arrays of various types have been recently developed which greatly facilitate the design of optoelectronic readers which function by scanning original documents in two dimensions. Typical of such solid state arrays are CCD (charge coupled device) and PDA (photodiode array) components. Such arrays comprise a large number of photosensitive elements, typically as many as 1728 elements, arranged in a line. The individual elements are sequentially electrically enabled or strobed in a predetermined direction for reading out electrical output signals corresponding to whether the corresponding portion of the original document has a density above or below a predetermined value.
These photosensor arrays are utilized in optoelectronic scanning readers by providing an optical system which focusses an image of a linear portion of an original document onto the array. The optical system is moved perpendicular to the linear portion of the document and thereby perpendicular to the array to effect vertical scan from top to bottom of the document. During the vertical scan, the photosensor elements of the array are enabled from left to right to effect horizontal scan. For each vertical scan, hundreds or thousands of horizontal scans must be performed to provide satisfactory image resolution. The electrical signals produced by the photosensor array are transmitted to a receiving device which reconstructs an image or hard copy of the document in response to the signals.
Although such scanning readers provide excellent performance, they suffer from a drawback regarding transmission speed where a single document must be scanned a number of times or a number of documents must be scanned rapidly in sequence. This is becsause the optical system must return to its original position prior to being moved for a subsequent vertical scan.
Where the return speed of the optical system is the same as the scan speed, the operating speed of the apparatus is unnecessarily reduced by a factor of two. Where provision is made to return the optical system at a speed which is significantly higher than the scan speed, the delicate components of the optical system are subjected to high rates of acceleration and deceleration which knock the optical system out of alignment in a relatively short period of time. If the document is scanned upwardly, or from bottom to top, the resulting reproduction will be a reversed, or mirror image of the original document.